Narrow SCSI - Expectations of Target's

Ashok Raj ashokr at
Thu Oct 9 15:23:16 PDT 1997

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* ashokr at (Ashok Raj)
[ Henry Wong wrote...]
> * From the T10 (formerly SCSI) Reflector (t10 at, posted by:
> * Henry Wong <hwong at>
> *
> Being in ASICs I seldom get out of the 1's & 0's world.
> I would like to hear what is the correct or prevelant way 
> to handle the following 'system' scenario (which is
> out of my league and handled by the firmware folks). 
> I would appreciate any & all feedback from both host
> adapter & target device folks!! 

I have not completely read this.. but from what i understand
looks like you are thinking that narrow is during selection phase.

but i beleive that during selection phase all ids are visible on 
the wide bus. being narrow on pon is only for data phases i think.

pls correct me if iam wrong.

>   1) A 'physically' WIDE Initiator with an ID=15
>   2) A 'physically' WIDE Target with an ID=1
>   3) System does not have SCAM & ID's are 
>      hard jumpered...let's keep this simple 
>   5) SCSI System is WIDE with Narrow only
>      devices mixed in ..but the Target doesn't 
>      know this yet.  
>   6) Target defaults to Narrow mode (let's
>      say after power-up) since it doesn't
>      know if the Bus if physically Narrow
>      or Wide yet 
>   7) The Initiator trys to select the Target
> ==> Now which of the three cases below is "correct"?
>  => Case1: The target is only monitoring the lower
>            SCSI Bus bits since it has reverted to 
>            Narrow and is not aware if the System is
>            a Wide or Narrow Bus.  It responds as if
>            it "thinks" it's in a SASI system because
>            it see's only one ID (it's own ID) on the
>            Bus.  Yes.. the target knows it's not a 
>            real SASI system since it gets the Identify
>            /ATN phase.  The target basically handles
>            the command w/o Disconnect.  After Cmd 
>            cmplt, switches to Wide (until negotiated
>            otherwise).
>  => Case2: The target being physically Wide always
>            monitors the high SCSI Bus bits even if 
>            it has reverted to Narrow mode...thus
>            will respond to the Initiator as if 
>            it were Wide and business as usual as
>            a "psuedo-wide device"...versus.. a 
>            Narrow device in a "real" Narrow bus.
>  => Case3: Have the target ignore the Selection 
>            all together since it doesn't know if
>            it must assume it is Narrow because it
>            may be on a Narrow Bus and that it should
>            not expect to be seeing a SASI system.
>            Also, knowing it won't know if it is a
>            SASI system until it responded to the 
>            Selection.   
> Perhaps "the" basic question is should the Target (knowing
> it's SCSI connector is physically Wide... ie. 68-pin 
> or 80-pin SCA-2) always observe and react accordingly
> to what is present on the High SCSI Bus byte.  In 
> the case of a Selection, always monitor bits 15-8+parity
> for proper ID & good parity before responding <irrespective
> of whether or not the Target is defaulting to Narrow
> Bus transfer mode>??  This also implies that the SCSI 
> system MUST terminate the High Bus if it intends to use
> and adapt a physically Wide device for a Narrow-only
> SCSI system.  Also, the High Bus Byte is ONLY ignored 
> when the Target PCB is physically Narrow (ie 50-pin 
> connectored)??
> Thanks!
> Regards, Henry        
> *
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